Fluorescent lamps may employ various combinations of three or four narrow band emitting phosphor materials for conversion of ultraviolet (UV) light to visible light. A “phosphor” is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum. Each phosphor material produces a red, a blue or a green color emission, and a combination of phosphor materials may be used, for example, in low pressure type mercury vapor discharge lamps. Crystalline inorganic compounds form an important class of phosphors that are of high chemical purity and of controlled composition to which small quantities of other elements (known as “activators”) have been added to convert them into efficient luminescent materials. Phosphors used in low pressure lamps mercury vapor discharge lamps convert ultraviolet (UV) radiation emitted by the excited mercury vapor to visible light.
Typically, a blue color emission phosphor exhibits an emission band in the wavelength range from about 430 nanometers (nm) to about 500 nm, a red color emission phosphor exhibits an emission band in the wavelength range from about 590 nm to about 670 nm, and a green color emission phosphor exhibits an emission band in the wavelength range from about 500 nm to about 600 nm. This phosphor combination produces efficient white light illumination. A blend of four narrow band emitting phosphor materials may include first and second green emitting phosphors having different visible emission spectrum, a third blue emitting phosphor, and a fourth red emitting phosphor. Some conventional phosphor combinations utilized a dual layer phosphor coating. For example, a thin layer of the tri-phosphor combination may be deposited on the surface of a basecoat of conventional calcium haloapatite phosphor material to produce the desired high light output with fair color rendition at a considerable cost savings for the overall phosphor combination. But the color rendering index (CRI) of the basecoat is typically about 70, and the tri-phosphor top layer CRI is typically about 83 and thus the CRI of the basecoat is much lower than that of the topcoat. The combination of the layers therefore results in a CRI of about 78.
Thus, a need exists for energy efficient lamps that provide a CRI of at least 87 while also reducing the cost of utilizing relatively expensive phosphor materials.